Optical glass scale linear encoders are known for measuring distances along a linear path by detecting light reflected from or transmitted through a linearly extending glass scale. The range of distances over which such encoders operate may be limited due to the difficulty of providing a sufficiently long glass scale with the requisite resolution and accuracy. Another type of linear encoder is a laser interferometer. While this type of encoder generally provides high accuracy and resolution, such encoders require coherent radiation, wavelength-stabilized lasers, are very expensive, depend upon incremental counting of interference cycles, and provide no zero-reference index as a datum point.
Electronic distance measurement systems (EDM) are known for measuring relatively long distances, such as in land surveying. These systems operate by generating an amplitude modulated light beam using a laser or a light emitting diode and determining the distance by the phase lag of the beam returning from a reflector. Phase lag is measured by counting time clock increments between crossovers from the transmitted beam to the received beam, providing resolutions around one millimeter. Coarse measurements are first made using low frequency modulation followed by higher modulating frequencies for the required resolution. Known EDM systems are not capable of achieving very high resolutions and higher resolutions are not needed for EDM use.